The present invention concerns quantum cascade semiconductor lasers. In the present application, quantum cascade semiconductor laser means a laser in which the gain is due to carriers performing inter-sub-band transitions in a semiconductor heterostructure.
A laser of this type is, for example, described in the document entitled xe2x80x9cBuried heterostructure quantum cascade lasers,xe2x80x9d published in SPIE Vol. 3284 0277 -788X98. its structure and the production process employed will be explained In detail in the following description, with reference to FIGS. 1a-1e.
Such lasers comprise a substrate of indium phosphide (InP), doped with silicon, on which is arranged a stack of very thin layers forming the laser waveguide. The stack has a prismatic structure, of substantially trapezoidal cross-section, of which the lower surface, forming the large base of the trapeze, rests on the substrate.
The lower and upper surfaces are each formed of layers of alloys of indium gallium arsenide (InGaAs) and of aluminium indium arsenide (AlInAs) forming a cladding.
A series of layers of aluminium indium arsenide (AlInAs) and indium gallium arsenide (InGaAs) forms the active zone of the laser, which is arranged between the two claddings.
To perform injection of an electric current in the active region, the laser also includes two electrodes. The first electrode is formed by the substrate. The second is embodied by means of a metallic layer placed above the substrate and in contact, on at least part of the upper surface of the stack. An electrically insulating layer, in InP, supports the metallic layer, except for the contact region.
The manufacture of such a laser involves using a substrate doped with InP, so as to make it conductive, to form the first electrode. Successive layers are then deposited, which form the stack. These layers are then attacked, conventionally, by photolithography. We thus obtain the prismatic structure of trapezoidal cross-section.
The assembly thus obtained is covered, by molecular-beam epitaxy, with a layer of InP covering the entire upper surface of the device at this stage of manufacturing. A further photolithography operation enables an opening to be made in the region covering the upper surface of the stack. The edges of the opening, consisting of the side edge of the electrically insulating layer, thus form the well at the bottom of which is located the upper surface of the stack. The metallic layer, for example chromium, is then deposited on the electrically insulating layer and on the upper surface of the stack, to form the second electrode.
By applying an electric voltage between the lower and upper surfaces of the stack, i.e. between the two electrodes, one injects into it an electric current which generates the laser radiation.
These lasers have the advantage of being able to be built to operate in a large part of the medium infrared spectrum (approximately 3 to 20 xcexcm wavelength) using the same basic semiconductor materials.
Lasers having similar structures, to which the present invention can be applied, have been described, in particular, in patent applications WO 99/00572 and FR 99 03845.
The operation of these lasers depends to a considerable extent on the temperature of the stack during emission. Now, when the laser is activated, a large proportion of the energy is transformed into heat. This heat is basically dissipated by diffusion in the substrate. The dissipation is relatively slow, which therefore considerably limits the conditions of use.
In the device manufactured as described above, it is advantageous for the electrically insulating layer to be as thick as possible, because InP is a good heat conductor. This makes it possible to avoid an excessive rise in temperature. Unfortunately, the thicker the layer, the harder it is to control etching with a view to forming the cutout designed to allow electrical contact between the electrode and the upper surface of the stack.
Moreover, the deposition of InP by molecular-beam epitaxy requires a cleaning operation under ultra-vacuum, which is difficult and costly.
In addition, the insulation function is based on the quality of a Schottky barrier, which is also hard to control.
The aim of the present invention is to permit the manufacture of lasers in which the electrically insulating layer ensures optimum heat dissipation, in a reproducible way and for a reasonable cost.
This aim is achieved thanks to the fact that the electrically insulating layer covers said lateral surfaces completely, without overlapping on the upper surface, and due to the fact that its thickness is equal to at least one third of the thickness of the stack. Such a thickness is sufficient to permit good thermal diffusion.
Although the stack is not very thick, it forms a protuberance which means that the assembly has a non-planar structure. Accordingly, the laser can be attached to a mounting only by its substrate. Now, the possibility of laser assembly on one or other surface indifferently simplifies the assembly operations. In cases where thermal dissipation plays a major role, it is advantageous to have a mounting that is highly heat-conducting, in direct contact with the electrically insulating layer. That is why, in a particular embodiment, the electrically insulating layer covers the substrate at least in the zone adjacent to the stack and has a thickness at least equal to that of the stack.
Advantageously, the thickness of the electrically insulating layer is equal to that of the stack. It is however foreseeable, for certain applications, to build an electrically insulating layer the thickness of which is greater than that of the stack. In this case too, thermal diffusion is improved and the flatness of the second surface is ensured.
In a particularly advantageous embodiment, the substrate is of semiconductor material doped to make it conducting.
Of the various types of quantum cascade semiconductor lasers, those having the following characteristics gave especially good results:
the substrate is formed of a plate in indium phosphide (InP), doped with silicon;
the claddings are each formed by layers of indium gallium arsenide alloy (InGaAs) and aluminium indium arsenide alloy;
the active region is formed of a series of layers of aluminium indium arsenide (AlInAs) and indium gallium arsenide (InGaAs), and
the electrically insulating layer is of InP.
In an initial variant, the indium phosphide forming the electrically insulating layer is of semi-insulating type, through iron doping.
In another variant, the indium phosphide forming the electrically insulating layer is of non-doped type.
The present invention also concerns a process for manufacturing a laser as described above. Extremely interesting results were obtained by applying a process involving the following operations:
deposition of layers designed to form the stack;
deposition of a masking layer;
etching of the masking layer outside of the zone designed to form the stack;
etching of the layers designed to form the stack outside of the masked zone;
metal organic chemical vapour deposition (MOCVD) of an electrically insulating layer on the non-masked parts, until a thickness at least equal to one-third of the stack thickness is reached;
removal of the masking layer, and
deposition of a layer covering in particular the stack on its upper surface.
Since only the upper surface of the prism is masked, the result is that the lateral surfaces are entirely covered by the electrically insulating layer while the upper surface is completely spared.
Advantageously, the operation of deposition of the electrically insulating layer is performed until a thickness substantially equal to the stack thickness is reached.
To avoid the layer of insulating material being deposited on the masking layer, it is advantageous for the latter layer to be made of silicon dioxide (SiO2).